We are continuing to describe the topography and function of the various receptor sites for molecular transport, including the response these structures make when occupied by a suitable substrate and co-substrate. Substrates are used with restricted or enhanced potentialities for producing these responses or for undergoing unrelated metabolic alteration. Some of these show enhanced access to the energetic gradients of the membrane and may thus serve to locate and identify these gradients. Cryptic gradients of H ion within the membrane appear to cause diamino acids to respond by flux asymmetry, to the degree that the alpha-amino group is able to yield a proton reversibly during transport. These effects will be studied by systematic alteration of pK values of the distal amino group, and by the use of agents that increase or decrease H ion flow. Study will be continued of the stimulating action of amino acids on an alkali-metal-independent cleavage of membrane ATP. Further study is planned of heterogeneity of amino acid absorption of epithelial tissues.